Polyimide resins prepared by the reaction of dianhydrides of aromatic tetracarboxylic acids with aromatic diamines are known products. See U.S. Pat. No. 3,179,634. Such polyimide resins are very high melting thermoplastic resins which have outstanding physical and chemical properties, including surprising stability to heat. Unfortunately, by reason of their high melting points, such resins are very difficult to fabricate by conventional techniques such as molding and extruding.
U.S. Pat. No. 3,845,018 and U.S. Pat. No. 3,897,349 disclose a somewhat related group of products prepared by reacting 1 molar portion of an aromatic diamine with 2 molar portions of a dianhydride of an aromatic tetracarboxylic acid. This product then is reacted with 2 molar portions of an acetylene substituted aromatic monoamine such as 3-aminophenylacetylene. The products are characterized as acetylene end-capped polyimide oligomers. These products can be fabricated by molding and/or laminating techniques to prepare thermoset resins having excellent heat stability. Unfortunately, the acetylene substituted aromatic monoamines are difficult to synthesize and very expensive.
An avenue explored to prepare reasonably priced polyimides has been to prepare resins from a solution of a mixture of a dialkyl ester of an aromatic tetracarboxylic acid, an aromatic diamine, and a monoalkyl ester of an unsaturated alicyclic dicarboxylic acid such as 5-norbornene-2,3-dicarboxylic acid monomethyl ester. Such products are described in NASA Tech Brief LEW-12738. Unfortunately, the performance properties of these resins do not equal the resins earlier referred to.
There remains in the art a need for polyimide resins having good physical properties, particularly heat stability, which can be readily fabricated by molding and laminating techniques, and which can be prepared at reasonable cost.